1. Field of the Invention
The present invention relates to a generation of magnetic fields for a nuclear magnetic resonance imaging, and more particularly, to a prevention of a cross-talk between coils used in the generation of magnetic fields.
2. Description of the Background Art
In general, in an NMR (Nuclear Magnetic Resonance) imaging apparatus, a desired pulse sequence is executed by generating gradient magnetic fields in x, y, and z directions by supplying prescribed amounts of currents to gradient magnetic field coils from a gradient magnetic field power source. Usually, the current to be supplied to each gradient magnetic field coil amounts to 150 to 200 A, so that there arises a cross-talk between the gradient magnetic field coils for generating gradient magnetic fields in different directions.
More specifically, a conventional apparatus for generating gradient magnetic fields in the NMR imaging apparatus has a configuration as shown in FIG. 1, which comprises: the gradient magnetic field toll system including an X-axis gradient magnetic field coil 107, a Y-axis gradient magnetic field coil 108, and a Z-axis gradient magnetic field coil 109; and the gradient magnetic field power source 101 including an X-channel amplifier (AMP.sub.x) 102 for outputting the current to be supplied to the X-axis gradient magnetic field coil 107, a Y-channel amplifier (AMP.sub.y) 103 for outputting the current to be supplied to the Y-axis gradient magnetic field coil 108, a Z-channel amplifier (AMP.sub.z) 104 for outputting the current to be supplied to the Z-axis gradient magnetic field coil 109 and a power source unit (PS) 105 for supplying necessary electric power to each of the amplifiers 102, 103, and 104; where the operations of the amplifiers 102, 103, and 104 are controlled by a controller 106 of the NMR imaging apparatus.
In this configuration of FIG. 1, the amplifiers 102, 103, and 104 are controlled to output the prescribed amounts of currents by the controller 106 according to the desired pulse sequence, such that the desired gradient magnetic fields can be generated by the gradient magnetic field coils 107, 108, and 109. Here, in this configuration of FIG. 1, cables for transmitting currents are provided in close proximity with each other at positions P1, P2, and P3 indicated in FIG. 1, so that the cross-talk can be caused between any two of the gradient magnetic field coils 107, 108, and 109.
Namely, when only an X-channel is supplied with the current having a waveform as indicated by G.sub.x in FIG. 2 while no current is supplied to Y- and Z-channels, for example, the currents having waveforms as indicated by G.sub.y and G.sub.z in FIG. 2 flow through the Y- and Z-channels, respectively, due to the influence of the current supplied to the X-channel. This is a phenomenon which has been known as the cross-talk. Note that the similar cross-talk can be caused in the X- and Z-channels when the current is supplied only to the Y-channel, or in the X- and Y-channels when the current is supplied only to the Z-channel.
Here, it is to be noted such a cross-talk can be caused by a coupling among cables as well as a coupling among coils. The cross-talk due to the coupling among cables can be prevented to some extent by shielding the cables and distancing the cables from each other, but such a measure is not applicable to the cross-talk due to the coupling among coils.
Thus, in a conventional gradient magnetic field power source used in the NMR imaging apparatus, the cross-talk is caused between the gradient magnetic field coils, and such a cross-talk can severely affect the encoding control or the phase control in a pulse sequence requiring a high precision such as that for the NMR angiography.
It is also to be noted that the similar problem of the cross-talk explained above for the gradient magnetic field coil system also arises in the shim coil system and the static magnetic field coil system, as well as among the static magnetic field coil system, the gradient magnetic field coil system, and the shim coil system.